1. Technical Field
This description generally relates to the field of electrical circuits and, in particular, to the design of charge pump circuits.
2. Description of the Related Art
Charge pumps are used to generate high bias voltages from a single low voltage supply. In the prior art, a typical charge pump consists of charge pump stages electrically connected in series with one another to form a chain. For applications where more than one bias voltage is needed, a separate charge pump chain is used to produce each of the required output voltages, each chain starting from the same single supplied voltage.
FIG. 1 shows an example of a typical charge pump chain 10. The charge pump chain 10 includes a plurality of charge pump stages 12-1, 12-2, 12-3, 12-4, a control component 13, a comparator 14, and a capacitor 15. The charge pump stages 12-1, 12-2, 12-3, 12-4 are electrically coupled in series; the output voltage of one stage serving as the input voltage of the next stage. The comparator 14 has a first input connected to the output of the final charge pump stage 12-4 and a second input connected to a reference voltage. The comparator 14 has an output connected to an input of the control component 13. An output of the control component 13 is connected to each of the charge pump stages 12-1, 12-2, 12-3, 12-4. Each of the charge pump stages 12-1, 12-2, 12-3, 12-4 is also provided with the supplied input voltage Vdd.
The control component 13 controls the position of a switch within each of the charge pump stages 12-1, 12-2, 12-3, 12-4. The position of the switches controlled by the control component 13 is selected depending on the output of the comparator 14, which compares the output voltage at the final stage 12-4 against the reference voltage.
Within an individual charge pump stage, a capacitor, a diode, and a switch are electrically configured so that by controlling the position of the switch relative to the position of the switches in neighboring stages an input voltage Vdd supplied to the stage is raised by an amount equal to Vdd. By electrically connecting a plurality of stages in series to form a chain, the supply voltage Vdd at the input of the initial stage 12-1 is stepped up incrementally by an amount Vdd at each stage. The overall output voltage for the charge pump chain 10 is taken at the output of the final stage 12-4 and equals Vdd+(n*Vdd), where n is the number of stages in the chain. The output at the final charge pump stage 12-4 is connected to ground through a decoupling capacitor 15, which smoothes the response of the output voltage as measured at the output of the final stage.
FIG. 1 also shows a charge pump typical in the prior art for an application requiring more than one output voltage level. In the prior art, to produce M output voltages, M number of charge pump chains is used. Each charge pump chain requires its own comparator and control circuitry to regulate the output voltage of that chain. In conventional charge pump designs, all M output voltages are produced starting from the same input voltage Vdd. A consequence of this design is that as applications with an increasing number of different voltage levels are needed, the number of chains and the number of stages needed increases. The increasing number of chains and stages leads to higher circuit complexity, the need for more wafer space on a semiconductor die, and higher power requirements.